Communication networks are known to employ laser sources in many different forms, from transponders to gain regions in amplifiers. Generally, these laser sources are characterized by an output bandwidth and output (or laser) frequency. In smaller-scale communication networks, like an enterprise network for example, a single frequency laser (850 nm) may be used to transmit data. For larger bandwidth applications, for example, links between metro-area networks (MANs), a dense wavelength division multiplexing (DWDM) system employs lasers over a range of frequencies. A DWDM system may include 100 channels, each channel emitting from a laser producing a different output frequency within a range of 1525-1565 nm. Output frequency and output bandwidth are the key design criteria for the network transponders housing these lasers. In the DWDM system in particular, control over frequency and bandwidth parameters is important to ensure proper channel spacing. Numerous industry standards, for example, pertain to output frequency and bandwidth spacing control.
Using tunable lasers in network devices is known. With tunable lasers, a manufacturer can choose from numerous, different output frequencies/wavelengths. Tunable lasers reduce manufacturing costs and eliminate the need for producing separate lasers of different output frequencies. Tunable lasers are also desirable because they are less susceptible to fabrication errors. If the desired output frequency is not exact, tuning the laser will take an otherwise useless output and tune it to a useful one. Some non-tunable/single frequency lasers may be tuned by temperature tuning, but the tuning is over a very small frequency range, one too small for tuning between different channels.
There are two primary ways of configuring a tunable laser. The first is via multi-segment Bragg reflectors (DBR). The second is via an external laser cavity. With respect to the former, each DBR serves as a highly reflective mirror cavity that produces a narrow-bandwidth output signal at a characteristic frequency. In some devices, selectively activating the DBR corresponding to the desired output frequency provides tuning. Each DBR is connected to a lead and separately activated. In other devices, like sampled-grating DBRs, electrical current is controlled to align reflection peaks between gratings, thereby tuning the device. Control over the electrical current is very difficult to achieve, however.
Multi-segment DBRs, while functional, have numerous drawbacks. These devices require individual control of each grating, which translates into increased control complexity. These devices have limited scalability. Additionally, because multi-segment DBRs must be formed to exacting layer width tolerances, more expensive fabrication techniques are required to create large numbers of DBRs and electrodes.
Another problem common to the multi-segment DBR tunable lasers is mode hopping. Here, the frequency of the output signal inadvertently hops from one output value to another output value. In the DWDM context, for example, such mode hopping would result in a multi-segment DBR tunable laser emitting at a frequency corresponding to one data channel and then inadvertently hopping to another frequency corresponding to another data channel. As a result of this mode hopping, a single data stream would be transmitted on different data channels, which is undesirable.
In an external cavity laser, a gain medium is placed between two mirrors, and one or both of the mirrors are moved to change the lasing cavity length. Additionally, a tuning element may be inserted into the cavity to select which of the modes will lase. Unfortunately, these external cavity lasers have moving parts that are expensive to fabricate. For example, some have proposed costly microelectronic mechanical systems (MEMS) devices to adjust the position of the moving components.
Other drawbacks exist. Moving parts must be reliably adjustable over extremely short distances, leaving them susceptible to performance degradation over time. Additionally, external cavity lasers are large in size and can result in undesirable coupling loss as energy is coupled into and out of the resonant cavity.